Faster-than-light neutrinos explained?

The detector at the Gran Sasso end of the OPERA experiment. Credit: {link url="http://operaweb.lngs.infn.it:2080/spip/spip.php?rubrique3&id_document=156#documents_portfolio"}OPERA{/link}

The faster-than-light neutrinos seen by the OPERA particle physics experiment last year may have just been explained. By a loose cable. I wish I was joking.

To back up a little, the OPERA collaboration based at the Gran Sasso laboratory underneath the mountain of the same name in Italy published a paper to pre-print server arxiv.org last September saying that they had seen neutrinos, a type of sub-atomic particle, travel faster than the speed of light. They recorded neutrinos, which had travelled from CERN, Geneva, through the Earth to Gran Sasso, Italy, arriving at the laboratory 60 nanoseconds earlier than they would had they travelled at the speed of light.

Since then, scientists around the world have been collectively scratching their heads and publishing papers that tended to fall into one of two categories: suggesting an error with the experiment (such as the clocks at the two laboratories not being synchronised properly), or suggesting an addition to the current theory of particle interactions that could explain the strange result (for example, a new dimension that the neutrinos could have skipped through to make their journey shorter – so they would have never actually travelled faster than light at any point).

But I don't think anyone expected it to be something as simple as this.

Today, Science is reporting that a fibre optic cable connecting a GPS receiver and an electronic card in a computer was loose. They go on:

After tightening the connection and then measuring the time it takes data to travel the length of the fibre, researchers found that the data arrive 60 nanoseconds earlier than assumed

This news (though still unconfirmed) rather casts a shadow over another recent explanation, involving something slightly less ridiculous.

In a paper published in journal Astronomy & Astrophysics, Claudio Germana of the Astronomical Observatory of Padova, Italy, suggests that there was a problem with the synchronisation of clocks at the two ends of the experiment. His calculations suggest that if the experiment had been run at a different time of year, the neutrinos would in fact have arrived 50 nanoseconds later than light.

I spoke to Carlo Contaldi, a physicist at Imperial College London, who last year published a paper on arxiv.org pointing out a possible problem with clock synchronisation, about the new paper. Though he thought the calculations and the large effect the calculations seemed to show were "interesting", he had some reservations:

[Germana] does not seem to mention the latest measurements that were carried out by OPERA in November 2012. Those showed a consistent value for the neutrino's time of flight as the previous results and it would be interesting to see how that time frame fits in with these corrections.

It's an interesting hypothesis though – and one that is easily testable by running the experiment at a different time of year.

This paper is just the latest in a long string of attempts to explain the faster-than-light neutrinos. For more of the explanations that have been offered over the last few months, have a look at a timeline I made that follows the story right from the beginning until now.

All of these papers could have been for nothing, of course, if the new report of a loose cable is true. It would be a little disappointing if this turns out to be the case. I'm going to reserve judgement for now, at least until the "sources familiar with the experiment" become something a little more concrete.

I'll be updating the above timeline as the story unfolds.

UPDATE: The Nature News Blog now has an official statement from OPERA, that says they have "identified two issues that could significantly affect the reported result" – you can read the full statement over there.

The OPERA collaboration has informed its funding agencies and host laboratories that it has identified two possible effects that could have an influence on its neutrino timing measurement. These both require further tests with a short pulsed beam. If confirmed, one would increase the size of the measured effect, the other would diminish it. The first possible effect concerns an oscillator used to provide the time stamps for GPS synchronizations. It could have led to an overestimate of the neutrino's time of flight. The second concerns the optical fibre connector that brings the external GPS signal to the OPERA master clock, which may not have been functioning correctly when the measurements were taken. If this is the case, it could have led to an underestimate of the time of flight of the neutrinos. The potential extent of these two effects is being studied by the OPERA collaboration. New measurements with short pulsed beams are scheduled for May.

So, there was a (possible) faulty cable that might have led to an underestimate of the time it took the neutrinos to reach Gran Sasso, which would led to an overestimate of their speed. But there was also another fault that might have led to an underestimate of the speed. Looks like we will have to wait for the new measurements in May to see just how much each of these faults contributed to the early arrival of the neutrinos and whether they can add up to the 60 nanoseconds to fully explain the result.

The views expressed are those of the author and are not necessarily those of Scientific American.

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